1. Field of the Invention
The present invention relates to an electric discharge wire cutting apparatus, and more particularly to an electric discharge wire cutting apparatus capable of automatically varying machining conditions dependent on the position of nozzles.
2. Description of the Related Art
It is widely known in the field of electric discharge wire cutting that the pressures of a machining liquid from nozzles are increased to cool the wire and to accelerate removal of machining chips, thereby increasing the machining rate. To this end, the nozzles are required to be located as closely to the workpiece as possible. One example of such an arrangement is disclosed in Japanese Utility Model Publication No. 52-123794.
FIG. 4 schematically shows the relationship between a workpiece and nozzles. A table 11 places a workpiece 12 between upper and lower nozzles 13 and 14, respectively. A machining liquid is poured from the upper and lower nozzles 13 and 14 into a machined hole (not shown) in the workpiece 12. The closer the nozzles are to the workpiece, the higher the pressure of the machining liquid, so that the workpiece can be machined at a higher speed under stricter conditions.
When a subtable 15 is used, as shown in FIG. 5, during a machining process, the lower nozzle 14 needs to be spaced a certain distance from the workpiece since the subtable 15 and the nozzle 14 would otherwise interfere with each other.
Moreover, as shown in FIG. 6, where a stack of thin workpieces are machined, the stacked workpieces 16 should be spaced a distance from the lower nozzle 14 because a workpiece holding screw 17 which holds the workpieces together would interfere with the lower nozzle 14. Therefore, the workpiece or workpieces may not be machined while the nozzles are being located closely to the workpiece or workpieces.
Machining conditions are generally determined while the nozzles are disposed closely to the workpiece as shown in FIG. 4. If the nozzles are spaced from the workpiece, the workpiece cannot properly be machined under normal machining conditions, and the wire may be broken or the machining accuracy may be lowered.
There is also a device for varying the rate of flow of a machining liquid dependent on the position of the nozzles. However, this device has not necessarily been useful since a considerable degree of skill is required on the part of the operator to determine an appropriate flow rate for the machining liquid.